A traditional monolithic RF base transceiver station (BTS) architecture is increasingly being replaced by a distributed BTS architecture in which the functions of the BTS are separated into two physically separate units—a baseband unit (BBU) and a remote radio head (RRH). The BBU performs baseband processing for the particular air interface that is being used to wirelessly communicate over the RF channel. The RRH performs radio frequency processing to convert baseband data output from the BBU to radio frequency signals for radiating from one or more antennas coupled to the RRH and to produce baseband data for the BBU from radio frequency signals that are received at the RRH through one or more antennas.
The RRH is typically installed near the BTS antenna(s), often at the top of a tower, and the BBU is typically installed in a more accessible location, often at the bottom of the tower. The BBU and the RRH are typically connected through one or more fiber optic links. The interface between the BBU and the RRH is defined by front-haul communication link standards such as the Common Public Radio Interface (CPRI) family of specifications, the Open Base Station Architecture Initiative (OBSAI) family of specifications, and the Open Radio Interface (ORI) family of specifications.
Wireless operators are under constant pressure to increase the speed, capacity and quality of their networks while reducing operating costs. As technologies evolve, that challenge is becoming increasingly difficult. One specific reason is that passive intermodulation (PIM) distortion products in an uplink band in an uplink path are having a more noticeable, detrimental effect on network performance, and thus are increasing network costs.
Intermodulation distortion (IMD) products are spurious signals in the uplink path, i.e. in an operating band of a receiver. Interference caused by IMD products decreases the sensitivity, and hence the dynamic range of the receiver. IMD products may be caused by active and passive sources, e.g. components of a transceiver system. The IMD products of active components in the transceiver system can be suppressed by techniques such as feed-forward linearization. However, PIM distortion (PIMD) products cannot be so removed.
PIMD is caused by nonlinearities in passive components arising from metal to metal contacts, and metal to insulator to metal contacts. For example, PIMD occurs at the metal to metal interfaces in antennas, cables and/or transceiver components. Sources of PIMD can also be found in nearby metal objects such as guy wires, anchors, roof flashings, and pipes. Also, rust, corrosion, loose connections, dirt, and oxidation may give rise to PIMD. Advanced wireless equipment requires increased dynamic range including higher sensitivity. The dynamic range and sensitivity of new technologies, like the Long Term Evolution (LTE) cellular networks, are limited by PIMD levels. For example, a 1 decibel drop in uplink sensitivity due to PIMD in the uplink band can reduce cell coverage by as much as 11 percent. Therefore, there is a need in the art for a technique to effectively and efficiently reduce PIMD to improve wireless equipment performance.